For example, Japanese Laid-open Patent Publication No. 2008-077071 (FIG. 8 of this publication) discusses, as an optical amplification module that amplifies light signals, an array optical amplification module including an optical amplifier array in which a plurality of semiconductor optical amplifiers (SOAs) are arrayed. Light signals of a plurality of channels are input from an optical fiber array provided on one side to the array optical amplification module, are optically amplified and switched by the SOAs, and are then output to optical fibers in an optical fiber array provided on the other side.
In the above-described array optical amplification module, reflection of light signals at end faces of the optical fibers and end faces of the SOAs (edge reflection) is suppressed to reduce optical coupling loss. For this reason, beams of light signals are obliquely incident on and emergent from the optical fiber array and the SOAs. Specifically, a plurality of (two) lenses are provided between the optical fiber array and the SOAs, and the beams of the light signals pass the lenses while being shifted by a predetermined offset amount from the centers of the lenses. Imaging is performed while the centers of the two lenses are substantially aligned by this confocal lens system, so that the beams of the light signals may become oblique to the end faces of the optical fibers (the end faces of the SOAs).
However, in the above-described related art, the optical coupling loss between the end faces of the optical fiber array and the end faces of the SOAs have wavelength dependence, and the optical coupling loss differs among a plurality of channels (wavelengths). The beams of the light signals at the end faces of the SOAs are optically coupled obliquely to the array direction, and the beams of the light signals at the end face of the optical fiber array are also optically coupled obliquely to the array direction. Thus, all reflecting directions of light overlap with the array direction, and this increases the wavelength dependence of the optical coupling loss.
To reduce the wavelength dependence of the optical coupling loss, the beams of the light signals are not inclined in the array direction at the end face of the optical fiber array, but are made incident in parallel with the axial direction of the optical fibers (perpendicular to the array direction). However, if the structure is simply changed so that the beams of the light signals are incident on the optical fibers in parallel, the beams are reflected by edge reflection of the optical fibers, and enter the SOAs. This degrades the characteristics of the SOAs. In the structure in which the beams of the light signals are incident in parallel with the axial direction of the optical fibers, edge reflection may be suppressed by obliquely forming the end faces of the optical fibers in the optical fiber array, but it is difficult to form the end faces of the optical fibers obliquely in the array direction.
FIG. 8 depicts the problem of edge reflection of the optical fiber array. When a plurality of optical fibers arranged in the array direction in the optical fiber array are collectively made oblique in the array direction, the positions of the end faces of the optical fibers (distances from the lenses) become different. For this reason, it is assumed that the fiber axis direction of an optical fiber array 800 is set parallel to an optical axis direction Z such that beams of light signals are incident in parallel, as illustrated in FIG. 8. In this case, in order for the positions of end faces 800aa to 800na of a plurality of optical fibers 800a to 800n in the optical fiber array 800 to be arranged in an array direction X at the same distance from a lens 801, each of the end faces 800aa to 800na of the optical fibers 800a to 800n is formed obliquely to the beams (the optical axis direction Z). This formation is troublesome and realistically difficult.